1. Field of the Invention
Embodiments of this invention relate generally to spindle motors of disk drives of the type generally used for storing digital data, and in particular embodiments to methods for generating low acoustic noise spindle motor commutation, and disk drive systems incorporating the same.
2. Description of Related Art
Modem computers require media in which digital data can be quickly stored and retrieved. Magnetizable (hard) layers on disks have proven to be a reliable media for fast and accurate data storage and retrieval. Disk drives that read data from and write data to hard disks have thus become popular components of computer systems. To access memory locations on a hard disk, a read/write head is positioned over the hard disk, and the hard disk rotates at an essentially constant velocity. By moving the read/write head radially over the rotating hard disk, all memory locations on the hard disk can be accessed.
A spindle motor coupled to a spindle and the hard disk is often used to rotate the hard disk at an essentially constant velocity. Spindle motors are often brushless DC motors, which develop torque by the interaction of radial magnetic fields produced by permanent magnets on the rotor and rotating radial magnetic fields produced by sequencing alternating currents in the multi-phase windings of the stator. Rotation of the rotor occurs as the rotor's magnetic fields, and hence its permanent magnets, "follow" the rotating magnetic fields of the stator.
A plurality of spindle motor drivers repetitively source and sink current through the stator windings to produce the alternating currents in the windings, with each winding's alternating current maintaining a fixed phase relationship with respect to the alternating currents in the other windings. This phasing of alternating current in the windings of a motor is known as commutation. Because each winding acts essentially as an inductor, as the current in each winding changes direction at the switching frequency of its corresponding spindle motor driver, the reduction of current flowing through the winding causes its magnetic field to collapse, producing a back electromotive force (EMF) across the winding. The back EMF causes a surge voltage and a corresponding surge current to appear at the spindle motor driver at the switching frequency of the spindle motor driver. The repetitive switching of current in the winding results in a fluctuating magnetic field, creating a voice-coil effect with small forces of attraction and repulsion between the winding and the adjacent housing causing slight vibrations and acoustic noise at the switching frequency.
Snubber circuits are often coupled between each winding and ground to reduce the surge voltages and currents caused by the back EMF. An example of a snubber circuit is disclosed in U.S. Pat. No. 4,334,254, incorporated herein by reference. Capacitive snubber circuits suppress voltage transients by supplying a discharge path between the inductive winding and ground, but require high charging currents when the spindle motor drivers are sourcing current into the windings. High charging current is undesirable because it increases the power dissipation of transistors in the spindle motor drivers, requiring more expensive larger-geometry transistors.
To minimize these charging currents, a resistance is often included in series with the snubber capacitance. The resistance provides a resistive charging path for the capacitor, slowing down the charging rate of the capacitor and decreasing the instantaneous current sourcing requirements of the spindle motor drivers. The reduced charging current rate is also beneficial because many spindle motor driver circuits rely on a specific current ramp profile during the initial application of power to the spindle motor to determine the start phase of commutation, and without the resistance the high charging currents would distort the current ramp profile and introduce commutation start-up errors.
However, the resistance also creates drawbacks. The resistance impedes the flow of current through the capacitance to ground during spindle motor commutation, reducing the ability of the capacitance to suppress voltage transients, surge currents, and acoustic noise.